Your doctor may need to change the doses of your medications or monitor you carefully for side effects. It may be difficult to remain firm despite wanting to have sex. Sildenafil treats erectile dysfunction by increasing blood flow to the penis during sexual stimulation. It can be dangerous to take sildenafil if you also take medicines called nitrates often given for chest pain. The dose can be increased to mg or decreased to 25mg depending on its effect. Common side effects include headache, facial flushing, indigestion, and dizziness.AdDon’t go to the pharmacy without checking prices at GoodRx® first. No commitment or fees. Find the lowest price in your neighborhood & use our free coupons to save up to 80%Service catalog: Prescription Coupons, Prescription Discounts, Online Doct. Examples of Viagra’s more commonly reported side effects include: headache. flushing (temporary warmth, redness, or deepening of skin color) stuffy nose. indigestion (upset stomach) mild vision. Who may not be able to take sildenafil. Sildenafil is not suitable for some people. To make sure it’s safe for you, speak to your doctor or pharmacist before taking sildenafil if you: have ever had an allergic reaction to sildenafil or any other medicine. have recently had a stroke, heart attack or a heart problem – your doctor should.